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While I'm partial to Newtonian telescopes for their incredible capability
versus cost, I must admit that one aspect of Newtonian and Dobsonian telescopes
that can put people off is the collimation process.
However, with a bit of setup and an alignment tool, the process need not
be all that difficult.
There are a couple of tools than can be applied, including the
collimation laser and the Cheshire eyepiece.
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Pluto or Bust Mouse Pad
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Prepare the Primary
If you've not done it yet, you need to place a notebook reenforcement ring
on the center of your primary -- or paint a dot on the center of your primary.
Don't worry, the secondary blocks off light from the center of the primary
anyway. So we can make best use of it as an aid to alignment.
A simple way to get your reenforcement ring centered is to use a compass
to draw a circle on some thin cardboard the diameter of your primary.
Then draw about a one inch diameter circle at the center of the primary-sized
circle.
Cut out the primary-sized cardboard circle, then cut out and remove the
circle in the center.
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A trick I've read about is to fold the cardboard cutout in half. Then
unfold and fold in half 90 degrees from the first fold. Unfold again, and
you have a cardboard circle with a cutout in the center, the fold lines
helping to identify the precise center of the hole.
Carefully place this cardboard circle gently on the top of your primary to
rest only on the outside edge of the primary. Don't push the cardboard
down onto the primary's surface.
Use the fold lines to help identify the center of the cutout circle, and
glue your reenforcement ring on the exposed primary at the center of the
hole.
This glued on ring can remain on your primary as a handy tool to aid in
subsequent collimations.
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Bay Watcher T-shirts
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The Initial View
The Cheshire eyepiece has a cross hair and a cutout in the side to
reflect light down the eyepiece tube as you're looking through it.
Chances are, you'll see something like this on your first peek if you've
never aligned your optics before, or if you've re-installed mirrors after a
cleaning.
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In this and the following images, the black circle and black
cross hairs represent the visible parts of the Cheshire eyepiece.
The
green circle represents the image of the secondary, which should be centered
in the Cheshire eyepiece view.
The blue circle represents the reflected image of the primary mirror,
which may initially be quite off center and not even entirely visible.
The black dot represents the notebook ring on the primary.
The red circle with cross hair represents the reflected image of
the Cheshire eyepiece.
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Collimation -- Secondary
The first step involves adjusting the secondary alignment until:
the secondary appears centered in the view
the reflection of the primary appears centered in the secondary
the reflected image of the reenforcement ring on the primary is centered on the
Cheshire eyepiece cross hair.
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This image shows the results of completing the secondary alignment. The
secondary is centered in the view, the reflected image of the primary appears
concentric with the secondary, and the center dot (or reenforcement ring)
appears centered on the Cheshire eyepiece cross hair.
Chances are, the reflected image of the Cheshire eyepiece (the red
cross hair) will not be centered.
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Collimation -- Primary
To center the reflected image of the Cheshire eyepiece, adjust the primary
alignment using the adjustment screws on the back of the primary mirror
cell.
Note that some mirror cells simply have 3 spring-loaded adjusting screws,
while others have 3 pairs of screws. When in pairs, usually one of the
pair is a tensioning screw that must be loosened, and the other is the actual
adjusting screw.
If you have the pairs of screws, loosen all three tensioning screws,
do the adjustment with the adjusting screws, then re-tighten the tensioning
screws.
Whichever method your primary cell uses, tweak them until you can co-align
the Cheshire reflection with the cross hairs built into the Cheshire
eyepiece.
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If you are successful in performing the previous two steps, you should see
something like the image above when you look through your Cheshire
eyepiece.
Note that the Cheshire, the secondary, and the image of the
primary all appear concentric.
The reenforcement ring you placed on the center of the primary appears
centered on the cross hairs of the Cheshire eyepiece.
The reflected image of the Cheshire eyepiece also appears centered on the
cross hairs.
If you see this, you will have a well collimated telescope that should
provide good images. Also, subsequent alignments will be less dramatic
unless you've had to remove the mirrors for cleaning.
If you wish, you can do a final tweak of the collimation on
your next outing by examining an inside and outside of focus star image.
You might need minor adjustments of your primary to get
an out of focus star image that looks like preciously centered concentric
rings.
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Aperture Fever Mouse Pad
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